Electrode and cell comprising the same

ABSTRACT

Provided are an electrode and a battery having superior charge-discharge cycle characteristics and capable of obtaining a higher energy density. The battery comprises a spirally wound electrode body ( 20 ) including a cathode ( 21 ) and an anode ( 22 ) spirally wound with a separator ( 23 ) in between. During charge, lithium metal is precipitated on the anode ( 22 ), so the capacity of the anode ( 22 ) is represented by the sum of a capacity component by insertion and extraction of lithium and a capacity component by precipitation and dissolution of the lithium metal. The anode ( 22 ) includes a mixture layer ( 22   b ) having a powdered anode active material, and the mixture layer ( 22   b ) has liquid absorption properties that when 1 μdm 3  of propylene carbonate is dropped on the mixture layer ( 22   b ) at 23° C., a contact angle that the mixture layer ( 22   b ) forms with a propylene carbonate drop becomes 10 degrees within 100 seconds. Thereby, the mixture layer ( 22   b ) is impregnated with an electrolyte solution quickly and uniformly, so lithium metal is uniformly precipitated on the whole mixture layer ( 22   b ).

TECHNICAL FIELD

[0001] The present invention relates to a battery comprising a cathode,an anode and an electrolyte solution, and an electrode used in thebattery.

BACKGROUND ART

[0002] In recent years, reduction in size and weight of portableelectric devices typified by cellular phones, PDAs (personal digitalassistants) or laptop computers has been vigorously pursued, and as partof the reduction, an improvement in energy density of batteries,specifically secondary batteries as power sources for the devices hasbeen strongly required.

[0003] One example of a secondary battery which can obtain a high energydensity is a lithium-ion secondary battery using a material capable ofinserting and extracting lithium (Li) such as a carbon material for ananode. The lithium-ion secondary battery is designed so that lithiuminserted into an anode material is always in an ion state, so the energydensity is highly dependent on the number of lithium ions capable ofbeing inserted into the anode material. Therefore, in the lithium-ionsecondary battery, it is expected that when the amount of insertion oflithium is increased, the energy density can be further improved.However, the amount of insertion of graphite, which is considered atpresent to be a material capable of the most effectively inserting andextracting lithium ions is theoretically limited to 372 mAh per gram onan electricity amount basis, and recently the amount of insertion ofgraphite has been approaching the limit by active development.

[0004] Another example of the secondary battery capable of obtaining ahigh energy density is a lithium secondary battery using lithium metalfor an anode, and using only precipitation and dissolution reactions oflithium metal for an anode reaction. In the lithium secondary battery, atheoretical electrochemical equivalent of the lithium metal is as largeas 2054 mAh/cm³, which is 2.5 times larger than that of graphite used inthe lithium-ion secondary battery, so it is expected that the lithiumsecondary battery can obtain a much higher energy density than thelithium-ion secondary battery. A large number of researchers have beenconducting research and development aimed at putting the lithiumsecondary battery to practical use (for example, Lithium Batteriesedited by Jean-Paul Gabano, Academic Press, 1983, London, New York).

[0005] However, the lithium secondary battery has a problem that when acharge-discharge cycle is repeated, a large decline in its dischargecapacity occurs, so it is difficult to put the lithium secondary batteryto practical use. The decline in the capacity occurs because the lithiumsecondary battery uses precipitation-dissolution reactions of thelithium metal in the anode. In accordance with charge and discharge, thevolume of the anode largely increases or decreases by the amount of thecapacity corresponding to lithium ions transferred between the cathodeand the anode, so the volume of the anode is largely changed, thereby itis difficult for a dissolution reaction and a recrystallization reactionof a lithium metal crystal to reversibly proceed. Further, the higherenergy density the lithium secondary battery achieves, the more largelythe volume of the anode is changed, and the more pronouncedly thecapacity declines. Moreover, falling off of precipitated lithium, or aloss of the precipitated lithium because the lithium forms a coatingwith an electrolyte solution is considered as a cause of the decline inthe capacity.

[0006] Therefore, the applicant of the invention have developed a novelsecondary battery in which the capacity of the anode includes a capacitycomponent by insertion and extraction of lithium and a capacitycomponent by precipitation and dissolution of lithium, and isrepresented by the sum of them (refer to International Publication No.WO 01/22519 A1). In the secondary battery, a carbon material capable ofinserting and extracting lithium is used for the anode, and lithium isprecipitated on a surface of the carbon material during charge. Thesecondary battery holds promise of improving charge-discharge cyclecharacteristics while achieving a higher energy density.

[0007] However, like the lithium secondary battery, the secondarybattery uses precipitation-dissolution reactions of lithium, so thesecondary battery has a problem that when a charge-discharge cycle isrepeated, a larger decline in the discharge capacity occurs, compared tothe lithium-ion secondary battery. In order to overcome the problem, itis considered that it is important to uniformly precipitate lithium onthe whole anode. For the purpose, it is required to contrive thestructure of the anode.

[0008] In a conventional lithium-ion secondary battery, a large numberof structural contrivances for improving characteristics have beenreported. For example, in Japanese Unexamined Patent ApplicationPublication No. Hei 10-270016, a method of improving liquid absorptionspeed in a surface of an electrode through forming a continuous shallowgroove on the surface of the electrode is reported, and in JapaneseUnexamined Patent Application Publication No. Hei 10-97863, a method ofimproving liquid absorption speed of an electrode through settingporosity of the electrode.

[0009] However, in the method disclosed in Japanese Unexamined PatentApplication Publication No. Hei 10-270016, the liquid absorption speedin the surface of the electrode can be improved, but it is difficult toimprove liquid absorption speed in the whole electrode, thereby it isdifficult to obtain sufficient characteristics. Further, in the methoddisclosed in Japanese Unexamined Patent Application Publication No. Hei10-97863, the liquid absorption speed of the electrode can be improved,but the volume density of the electrode is sacrificed, so it isdifficult to obtain a high energy density.

[0010] In view of the foregoing, it is an object of the invention toprovide an electrode and a battery both having superior charge-dischargecycle characteristics and capable of obtaining a higher energy density.

DISCLOSURE OF THE INVENTION

[0011] An electrode according to the invention comprises: a mixturelayer including a powdered electrode active material, wherein themixture layer has liquid absorption properties that when 1 μdm³ ofpropylene carbonate is dropped on the mixture layer at 23° C., a contactangle that the mixture layer forms with a propylene carbonate dropbecomes 10 degrees or less within 100 seconds.

[0012] A battery according to the invention comprises: a cathode; ananode; and an electrolyte solution, wherein at least either the cathodeor the anode comprises: a mixture layer including a powdered electrodeactive material, wherein the mixture layer has liquid absorptionproperties that when 1 μdm³ of propylene carbonate is dropped on themixture layer at 23° C., a contact angle that the mixture layer formswith a propylene carbonate drop becomes 10 degrees or less within 100seconds.

[0013] In the electrode and the battery according to the invention, themixture layer of the electrode has the liquid absorption properties thatwhen 1 μdm³ of propylene carbonate is dropped on the mixture layer at23° C., the contact angle that the mixture layer forms with thepropylene carbonate drop becomes 10 degrees within 100 seconds, so themixture layer is impregnated with the electrolyte solution quickly anduniformly. Therefore, superior charge-discharge cycle characteristicsand a higher energy density can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a sectional view of a secondary battery according to anembodiment of the invention;

[0015]FIG. 2 is an enlarged sectional view of a part of a spirally woundelectrode body in the secondary battery shown in FIG. 1;

[0016]FIG. 3 is an illustration for describing a contact angle that ananode forms with a propylene carbonate drop;

[0017]FIG. 4 is a plot showing a relationship between volume density andliquid absorption time in a mixture layer of an anode according toExamples 1-1 through 1-6 of the invention;

[0018]FIG. 5 is a plot showing a relationship between volume density andliquid absorption time in a mixture layer of an anode according toExamples 1-7 through 1-12 of the invention; and

[0019]FIG. 6 is a plot showing a relationship between volume density andliquid absorption time in a mixture layer of an anode according toExamples 1-13 through 1-18 of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0020] Preferred embodiments of the invention will be described in moredetail below referring to the accompanying drawings.

[0021]FIG. 1 shows a sectional view of a secondary battery according toan embodiment of the invention. The secondary battery is a so-calledcylindrical type, and comprises a spirally wound electrode body 20including a strip-shaped cathode 21 and a strip-shaped anode 22 spirallywound with a separator 23 in between in a substantially hollowcylindrical-shaped battery can 11. The battery can 11 is made of, forexample, nickel (Ni)-plated iron (Fe). An end portion of the battery can11 is closed, and the other end portion thereof is opened. Anelectrolyte solution is injected into the battery can 11 to impregnatethe separator 23 with the electrolyte solution. Moreover, a pair ofinsulating plates 12 and 13 are disposed so that the spirally woundelectrode body 20 is sandwiched therebetween in a directionperpendicular to a spirally wound peripheral surface.

[0022] In the opened end portion of the battery can 11, a battery cover14 and, a safety valve mechanism 15 and a PTC device (positivetemperature coefficient device) 16 disposed inside the battery cover 14are mounted through caulking by a gasket 17, and the interior of thebattery can 11 is sealed. The battery cover 14 is made of, for example,the same material as that of the battery can 11. The safety valvemechanism 15 is electrically connected to the battery cover 14 throughthe PTC device 16, and when internal pressure in the battery increasesto higher than a certain extent due to an internal short circuit orexternal application of heat, a disk plate 15 a is flipped so as todisconnect the electrical connection between the battery cover 14 andthe spirally wound electrode body 20. When a temperature rises, the PTCdevice 16 limits a current by an increased resistance, thereby resultingin preventing abnormal heat generation by a large current. The PTCdevice 16 is made of, for example, barium titanate semiconductorceramic. The gasket 17 is made of, for example, an insulating material,and its surface is coated with asphalt.

[0023] The spirally wound electrode body 20 is wound around, forexample, a center pin 24. A cathode lead 25 made of aluminum (Al) or thelike is connected to the cathode 21 of the spirally wound electrode body20, and an anode lead 26 made of nickel or the like is connected to theanode 22. The cathode lead 25 is welded to the safety valve mechanism 15so as to be electrically connected to the battery cover 14, and theanode lead 26 is welded and electrically connected to the battery can11.

[0024]FIG. 2 shows an enlarged view of a part of the spirally woundelectrode body 20 shown in FIG. 1. The cathode 21 has, for example, astructure in which a mixture layer 21 b is disposed on both sides of acurrent collector 21 a having a pair of surfaces facing each other. Inaddition, the mixture layer 21 b may be disposed on only one side of thecurrent collector 21 a, although it is not shown. The current collector21 a is made of, for example, metal foil such as aluminum foil, nickelfoil or stainless foil with a thickness of approximately from 5 μm to 50μm. The mixture layer 21 b has, for example, a thickness of 80 μm to 250μm, and includes a cathode active material as an electrode activematerial. Further, when the mixture layer 21 b is disposed on both sidesof the current collector 21 a, the thickness of the mixture layer 21 bmeans the total thickness thereof.

[0025] The mixture layer 21 b requires to include a lithium contentequal to a charge-discharge capacity of 280 mAh or over per gram of ananode active material, which will be described later, in a steady state(for example, after a charge-discharge cycle is repeated 5 times or so),and more preferably to include a lithium content equal to acharge-discharge capacity of 350 mAh or over per gram of the anodeactive material. Therefore, as the cathode active material, a compoundincluding lithium such as lithium oxide, lithium sulfide or anintercalation compound including lithium is suitable, and a mixtureincluding two or more kinds selected from them may be used. Morespecifically, in order to achieve a higher energy density, a lithiumcomplex oxide or an intercalation compound including lithium representedby a general formula Li_(x)MO₂ is preferable. In the formula, as M, oneor more kinds of transition metals, more specifically at least one kindselected from the group consisting of cobalt (Co), nickel, manganese(Mn), iron, aluminum, vanadium (V) and titanium (Ti) is preferable. Thevalue of x depends upon a charge-discharge state of the battery, and isgenerally within a range of 0.05≦x≦1.10. In addition, LiMn₂O₄ having aspinel crystal structure, LiFePO₄ having an olivine crystal structure,or the like is preferable, because a higher energy density can beobtained.

[0026] It is not necessarily required to supply lithium from the cathodeactive material, and, for example, lithium may be supplied throughbonding lithium metal or the like to the anode 22 to add lithium ions inthe battery. In other words, a lithium content equal to acharge-discharge capacity of 280 mAh or over per gram of the anodeactive material may be included in a battery system. The lithium contentin the battery system can be determined through measuring the dischargecapacity of the battery.

[0027] The above cathode active material is prepared through thefollowing steps. For example, after a carbonate, a nitrate, an oxide ora hydroxide including lithium, and a carbonate, a nitrate, an oxide or ahydroxide including a transition metal are mixed so as to have a desiredcomposition, and the mixture is pulverized, the pulverized mixture isfired at a temperature ranging from 600° C. to 1000° C. in an oxygenatmosphere, thereby the cathode material is prepared.

[0028] The mixture layer 21 b includes, for example, an electronicconductor, and may further include a binder, if necessary. Examples ofthe electronic conductor include carbon materials such as graphite,carbon black and ketjen black, and one kind or a mixture of two or morekinds selected from them is used. In addition to the carbon materials,any electrically conductive material such as a metal material or aconductive high molecular weight material may be used. Examples of thebinder include synthetic rubber such as styrene butadiene rubber,fluorine rubber or ethylene propylene diene rubber, or a high molecularweight material such as polyvinylidene fluoride, and one kind or amixture including two or more kinds selected from them is used.

[0029] Like the cathode 21, the anode 22 has, for example, a structurein which a mixture layer 22 b is disposed on both sides of a currentcollector 22 a having a pair of surfaces facing each other. The mixturelayer 22 b may be disposed on only one side of the current collector 22a, although it is not shown. The current collector 22 a is made of, forexample, metal foil having excellent electrochemical stability, electricconductivity and mechanical strength such as copper foil, nickel foil orstainless foil.

[0030] The mixture layer 22 b includes an anode active material as apowdered electrode active material, and may further include, forexample, the same binder as that included in the mixture layer 21 b, ifnecessary. The mixture layer 22 b has a thickness of, for example, 60 μmto 250 μm. When the mixture layer 22 b is disposed on both sides of thecurrent collector 22 a, the thickness of the mixture layer 22 b meansthe total thickness thereof.

[0031] As the powdered anode active material, an anode material capableof inserting and extracting lithium as light metal is cited. In thedescription, insertion and extraction of light metal mean that lightmetal ions are electrochemically inserted and extracted without losingtheir ionicity. It includes not only the case where inserted lithiummetal exists in a perfect ion state but also the case where the insertedlithium metal exists in an imperfect ion state. These cases include, forexample, insertion by electrochemical intercalation of light metal ionsinto graphite. Further, insertion of the light metal into an alloyincluding an intermetallic compound, or insertion of the light metal byforming an alloy can be included.

[0032] As the anode material capable of inserting and extractinglithium, for example, a carbon material such as graphite,non-graphitizable carbon or graphitizing carbon is used. These carbonmaterials are preferable, because a change in the crystalline structurewhich occurs during charge and discharge is extremely small, so a highercharge-discharge capacity and superior charge-discharge cyclecharacteristics can be obtained. Further, graphite is more preferable,because its electrochemical equivalent is large, and a higher energydensity can be obtained.

[0033] For example, graphite with a true density of 2.10 g/cm³ or overis preferable, and graphite with a true density of 2.18 g/cm³ or over ismore preferable. In order to obtain such a true density, a c-axiscrystalline thickness of a (002) plane is required to be 14.0 nm orover. Moreover, the spacing of (002) planes is preferably less than0.340 nm, and more preferably within a range from 0.335 nm to 0.337 nm.

[0034] The graphite may be natural graphite or artificial graphite. Theartificial graphite can be obtained through the following steps, forexample. An organic material is carbonized, and high-temperature heattreatment is carried out on the carbonized organic material, then theorganic material is pulverized and classified so as to obtain theartificial graphite. The high-temperature treatment is carried out inthe following steps. For example, the organic material is carbonized at300° C. to 700° C. in an airflow of an inert gas such as nitrogen (N₂),if necessary, and then the temperature rises to 900° C. to 1500° C. at arate of 1° C. to 100° C. per minute, and the temperature is kept for 0to 30 hours to calcine the organic material, then the organic materialis heated to 2000° C. or over, preferably 2500° C. or over, and thetemperature is kept for an adequate time.

[0035] As the organic material as a starting material, coal or pitch canbe used. Examples of the pitch include a material which can be obtainedby distillation (vacuum distillation, atmospheric distillation or steamdistillation), thermal polycondensation, extraction, and chemicalpolycondensation of tars which can be obtained by thermally crackingcoal tar, ethylene bottom oil, crude oil or the like at hightemperature, asphalt or the like, a material produced duringcarbonization of wood, a polyvinyl chloride resin, polyvinyl acetate,polyvinyl butyrate, or a 3,5-dimethylphenol resin. These coals andpitches exist in a liquid state around at 400° C. at the highest duringcarbonization, and by keeping the coals and pitches at the temperature,aromatic rings are condensed and polycycled, so the aromatic rings arealigned in a stacking arrangement. After that, a solid carbon precursor,that is, semi-coke is formed at approximately 500° C. or over(liquid-phase carbonization process).

[0036] Moreover, as the organic material, a condensed polycyclichydrocarbon compound such as naphthalene, phenanthrene, anthracene,triphenylene, pyrene, perylene, pentaphene or pentacene, a derivativethereof (for example, carboxylic acid of the above compound, carboxylicacid anhydride, carboxylic acid imide), or a mixture thereof can beused. Further, a condensed heterocyclic compound such as acenaphthylene,indole, isoindole, quinoline, isoquinoline, quinoxaline, phthalazine,carbazole, acridine, phenazine and phenanthridine, a derivative thereof,or a mixture thereof can be used.

[0037] In addition, pulverization may be carried out before or aftercarbonization and calcination, or during a rise in temperature beforegraphitization. In these cases, the material in powder form is heatedfor graphitization in the end. However, in order to obtain graphitepowder with a higher bulk density and a higher fracture strength, it ispreferable that after the material is molded, the molded material isheated, then the graphitized molded body is pulverized and classified.

[0038] For example, in order to form the graphitized molded body, aftercoke as a filler and binder pitch as a molding agent or a sinteringagent are mixed and molded, a firing step in which the molded body isheated at a low temperature of 1000° C. or less and a step ofimpregnating the fired body with the molten binder pitch are repeatedseveral times, and then the body is heated at high temperature. Thebinder pitch with which the fired body is impregnated is carbonized bythe above heat treatment process so as to be graphitized. In this case,the filler (coke) and the binder pitch are used as the materials, sothey are graphitized as a polycrystal, and sulfur or nitrogen includedin the materials is generated as a gas during the heat treatment,thereby minute pores are formed in a path of the gas. Therefore, thereare some advantages that insertion and extraction of lithium proceedmore easily by the pores, and industrial processing efficiency ishigher. Further, as the material of the molded body, a filler havingmoldability and sinterability may be used. In this case, the binderpitch is not required.

[0039] The non-graphitizable carbon having the spacing of the (002)planes of 0.37 nm or over and a true density of less than 1.70 g/cm³,and not showing an exothermic peak at 700° C. or over in a differentialthermal analysis (DTA) in air is preferable.

[0040] Such non-graphitizable carbon can be obtained, for example,through heating the organic material around at 1200° C., and pulverizingand classifying the material. Heat treatment is carried out through thefollowing steps. After, if necessary, the material is carbonized at 300°C. to 700° C. (solid phase carbonization process), a temperature risesto 900° C. to 1300° C. at a rate of 1° C. to 100° C. per minute, and thetemperature is kept for 0 to 30 hours. Pulverization may be carried outbefore, or after carbonization or during a rise in temperature.

[0041] As the organic material as a starting material, for example, apolymer or a copolymer of furfuryl alcohol or furfural, or a furan resinwhich is a copolymer including macromolecules thereof and any otherresin can be used. Moreover, a conjugated resin such as a phenolicresin, an acrylic resin, a vinyl halide resin, a polyimide resin, apolyamide imide resin, a polyamide resin, polyacetylene orpolyparaphenylene, cellulose or a derivative thereof, coffee beans,bamboos, crustacea including chitosan, kinds of bio-cellulose usingbacteria can be used. Further, a compound in which a functional groupincluding oxygen (O) is introduced into petroleum pitch with, forexample, a ratio H/C of the number of atoms between hydrogen (H) andcarbon (C) of from 0.6 to 0.8 (that is, an oxygen cross-linked compound)can be used.

[0042] The oxygen content in the compound is preferably 3% or over, andmore preferably 5% or over (refer to Japanese Unexamined PatentApplication Publication No. Hei 3-252053). The oxygen content has aninfluence upon the crystalline structure of a carbon material, and whenthe content is the above value or over, the physical properties of thenon-graphitizable carbon can be improved, thereby the capacity of theanode 22 can be improved. Moreover, the petroleum pitch can be obtained,for example, by distillation (vacuum distillation, atmosphericdistillation or steam distillation), thermal polycondensation,extraction, and chemical polycondensation of tars obtained throughthermally cracking coal tar, ethylene bottom oil or crude oil at hightemperature, asphalt or the like. Further, as a method of forming anoxygen cross-link, for example, a wet method of reacting a solution suchas nitric acid, sulfuric acid, hypochlorous acid or a mixture thereofand petroleum pitch, a dry method of reacting an oxidizing gas such asair or oxygen and petroleum pitch, or a method of reacting a solidreagent such as sulfur, ammonium nitrate, ammonium persulfate or ferricchloride and petroleum pitch can be used.

[0043] The organic material as the starting material is not limited tothem, and any other organic material which can become non-graphitizablecarbon through the solid-phase carbonization by an oxygen cross-linkingprocess or the like may be used.

[0044] As the non-graphitizable carbon, in addition to thenon-graphitizable carbon formed of the above organic material as astarting material, a compound including phosphorus (P), oxygen andcarbon as main components which is disclosed in Japanese UnexaminedPatent Application Publication No. Hei 3-137010 is preferable, becausethe above-described parameters of physical properties are exhibited.

[0045] As the anode material capable of inserting and extractinglithium, a metal element or a metalloid element capable of forming analloy with lithium, or an alloy of the metal element or the metalloidelement, or a compound of the metal element or the metalloid element iscited. They are preferable because a higher energy density can beobtained, and it is more preferable to use them with a carbon material,because a higher energy density and superior cycle characteristics canbe obtained. In the description, the alloy means not only an alloyincluding two or more kinds of metal elements but also an alloyincluding one or more kinds of metal elements and one or more kinds ofmetalloid elements. As the composition of the alloy, a solid solution, aeutectic (eutectic mixture), an intermetallic compound or thecoexistence of two or more kinds selected from them is cited.

[0046] Examples of such a metal element or a metalloid element includetin (Sn), lead (Pb), aluminum, indium (In), silicon (Si), zinc (Zn),antimony (Sb), bismuth (Bi), cadmium (Cd), magnesium (Mg), boron (B),gallium (Ga), germanium (Ge), arsenic (As), silver (Ag), zirconium (Zr),yttrium (Y) and hafnium (Hf). As an alloy or a compound thereof, forexample, an alloy or a compound represented by a chemical formulaMa_(s)Mb_(t)Li_(u) or a chemical formula Ma_(p)Mc_(q)Md_(r) is cited. Inthese chemical formulas, Ma represents at least one kind selected frommetal elements and metalloid elements which can form an alloy or acompound with lithium, Mb represents at least one kind selected frommetal elements and metalloid elements except for lithium and Ma, Mcrepresents at least one kind selected from nonmetal elements, and Mdrepresents at least one kind selected from metal elements and metalloidelements except for Ma. Further, the values of s, t, u, p, q and r ares>0, t≧0, u≧0, p>0, q>0 and r≧0, respectively.

[0047] Among them, a metal element or a metalloid element selected fromGroup 4B elements in the short form of the periodic table of theelements, or an alloy thereof or a compound thereof is preferable, andsilicon or tin, or an alloy thereof or a compound thereof is morepreferable. They may have a crystalline structure or an amorphousstructure.

[0048] Specific examples of such an alloy or such a compound includeLiAl, AlSb, CuMgSb, SiB₄, SiB₆, Mg₂Si, Mg₂Sn, Ni₂Si, TiSi₂, MoSi₂,CoSi₂, NiSi₂, CaSi₂, CrSi₂, Cu₅Si, FeSi₂, MnSi₂, NbSi₂, TaSi₂, VSi₂,WSi₂, ZnSi₂, SiC, Si₃N₄, Si₂N₂O, SiO_(v) (0<v≦2), SnO_(w) (0<w≦2),SnSiO₃, LiSiO, LiSnO and the like.

[0049] As the anode material capable of inserting and extractinglithium, other metal compounds or high molecular weight materials arecited. As the metal compounds, oxides such as iron oxide, rutheniumoxide and molybdenum oxide, LiN₃, and the like are cited, and as thehigh molecular weight materials, polyacetylene, polyaniline, polypyrroleand the like are cited.

[0050] Among these anode active materials, specifically an anode activematerial in which an insertion reaction of lithium ions are active ispreferably used, and an anode active material with a charge-dischargepotential relatively close to that of lithium metal is more preferablyused.

[0051] Moreover, in the secondary battery, during charge, precipitationof lithium metal on the anode 22 begins at a point where an open circuitvoltage (that is, battery voltage) is lower than an overcharge voltage.In other words, in a state where the open circuit voltage is lower thanthe overcharge voltage, the lithium metal is precipitated on the anode22, so the capacity of the anode 22 is represented by the sum of acapacity component by insertion and extraction of lithium and a capacitycomponent by precipitation and dissolution of the lithium metal.Therefore, in the secondary battery, both of the anode material capableof inserting and extracting lithium and the lithium metal have afunction as an anode active material, and the anode material capable ofinserting and extracting lithium is a base material when the lithiummetal is precipitated.

[0052] The overcharge voltage means an open circuit voltage when thebattery is overcharged, and indicates, for example, a voltage higherthan the open circuit voltage of a battery “fully charged” described inand defined by “Guideline for safety assessment of lithium secondarybatteries” (SBA G1101) which is one of guidelines drawn up by JapanStorage Battery industries Incorporated (Battery Association of Japan).In other words, the overcharge voltage indicates a higher voltage thanan open circuit voltage after charge by using a charging method usedwhen a nominal capacity of each battery is determined, a standardcharging method or a recommended charging method. More specifically, thesecondary battery is fully charged, for example, at an open circuitvoltage of 4.2 V, and the lithium metal is precipitated on a surface ofthe anode material capable of inserting and extracting lithium in a partof the range of the open circuit voltage of from 0 V to 4.2 V.

[0053] Thereby, in the secondary battery, a higher energy density can beobtained, and cycle characteristics and high-speed chargecharacteristics can be improved, because of the following reason. Thesecondary battery is equivalent to a conventional lithium secondarybattery using lithium metal or a lithium alloy for the anode in a sensethat the lithium metal is precipitated on the anode 22. However, in thesecondary battery, the lithium metal is precipitated on the anodematerial capable of inserting and extracting lithium, thereby it isconsidered that the secondary battery has the following advantages.

[0054] Firstly, in the conventional lithium secondary battery, it isdifficult to uniformly precipitate the lithium metal, which causesdegradation in cycle characteristics; however, the anode materialcapable of inserting and extracting lithium generally has a largesurface area, so in the secondary battery, the lithium metal can beuniformly precipitated. Secondly, in the conventional lithium secondarybattery, a change in volume according to precipitation and dissolutionof the lithium metal is large, which also causes degradation in thecycle characteristics; however, in the secondary battery, the lithiummetal is precipitated in gaps between particles of the anode materialcapable of inserting and extracting lithium, so a change in volume issmall. Thirdly, in the conventional lithium secondary battery, thelarger the amount of precipitation and dissolution of the lithium metalis, the bigger the above problem becomes; however, in the secondarybattery, insertion and extraction of lithium by the anode materialcapable of inserting and extracting lithium contributes to acharge-discharge capacity, so in spite of a large battery capacity, theamount of precipitation and dissolution of the lithium metal is small.Fourthly, when the conventional lithium secondary battery is quicklycharged, the lithium metal is more nonuniformly precipitated, so thecycle characteristics are further degraded. However, in the secondarybattery, in an initial charge, lithium is inserted into the anodematerial capable of inserting and extracting lithium, so the secondarybattery can be quickly charged.

[0055] In order to more effectively obtain these advantages, forexample, it is preferable that at the maximum voltage before the opencircuit voltage becomes an overcharge voltage, the maximum capacity ofthe lithium metal precipitated on the anode 22 is from 0.05 times to 3.0times larger than the charge capacity of the anode material capable ofinserting and extracting lithium. When the amount of precipitation ofthe lithium metal is too large, the same problem as that which occurs inthe conventional lithium secondary battery arises, and when the amountis too small, the charge-discharge capacity cannot be sufficientlyincreased. Moreover, for example, the discharge capacity of the anodematerial capable of inserting and extracting lithium is preferably 150mAh/g or over. The larger the ability to insert and extract lithium is,the smaller the amount of precipitation of the lithium metal relativelybecomes. In addition, the charge capacity of the anode material isdetermined by the quantity of electricity when the battery with theanode made of the anode material as an anode active material and thelithium metal as a counter electrode is charged by a constant-currentconstant-voltage method until reaching 0 V. For example, the dischargecapacity of the anode material is determined by the quantity ofelectricity when the battery is subsequently discharged for 10 hours ormore by a constant-current method until reaching 2.5 V.

[0056] Moreover, in the secondary battery, when 1 μdm³ of propylenecarbonate is dropped on the mixture layer 22 b of the anode 22 at 23°C., the mixture layer 22 b has liquid absorption properties that acontact angle θ that the mixture layer 22 b forms with a propylenecarbonate drop P (refer to FIG. 3) becomes 10 degrees or less within 100seconds. In the secondary battery, precipitation-dissolution reactionsof lithium proceed only in a portion where the electrolyte solution isheld, and a portion where lithium is precipitated depends upon speedwith which the mixture layer 22 b is impregnated with the electrolytesolution, so the mixture layer 22 b with such a structure is impregnatedwith the electrolyte solution quickly and uniformly, thereby lithiummetal can be uniformly precipitated. In other words, it is consideredthat lithium metal can be prevented from being nonuniformlyprecipitated, thereby falling off of precipitated lithium or a loss oflithium due to a reaction between the precipitated lithium and theelectrolyte solution can be prevented.

[0057] As a method of impregnating the mixture layer 22 b with theelectrolyte solution quickly and uniformly, it can be considered toincrease porosity, but when the porosity is increased, the volumedensity decreases, so it is difficult to obtain a high energy density.Moreover, it is considered that the speed with which the mixture layer22 b is impregnated with the electrolyte solution is not proportional toall air gaps in the electrode, and depends upon the distribution of airgaps. For example, in spite of the fact that each air gap is small, whenair gaps are uniformly and continuously distributed on the wholeelectrode, the mixture layer 22 b can be quickly impregnated with theelectrolyte solution. On the other hand, in spite of the fact that thevolume of each air gap is large, when the air gaps are distributedunevenly and intermittently, it is difficult to quickly impregnate themixture layer 22 b with the electrolyte solution. Therefore, in theembodiment, the liquid absorption properties of the mixture layer 22 bis defined as described above, so even if the volume density increases,the mixture layer 22 b can be impregnated with the electrolyte solutionquickly and uniformly. In order to obtain a sufficient energy density,the volume density of the mixture layer 22 b is preferably 1.5 g/cm³ orover, more preferably 1.65 g/cm³, and most preferably 1.75 g/cm³. When acarbon material is used as the anode active material, the theoreticaltrue density of graphite as the carbon material is 2.265 g/cm³, so thevolume density of the mixture layer 22 b is actually 2.2 g/cm³ or less.

[0058] In order for the mixture layer 22 b to obtain the above liquidabsorption properties, air gaps formed in the anode active material arerequired to be reduced in size and be uniformly distributed. For thepurpose, the powdered anode active material preferably has a sphericalshape so that the anode active material can be most closely packed inthe mixture layer 22 b. More specifically, the average value ofcircularity (that is, average circularity) of a shadow of the anodeactive material in the mixture layer 22 b is preferably 0.7 or over.

[0059] The circularity is defined by a ratio between the area of ashadow and the area of a region where the center of a circle with anarea equal to the area of the shadow (the diameter of the circle iscalled effective diameter) and the shadow are overlapped when the centerof the circle is placed in the center of the shadow, and is representedby Mathematical Formula 1.

Circularity=A/S  (Mathematical Formula 1)

[0060] In the formula, A represents the area of the region where thecircle with an area equal to the area of the shadow and the shadow areoverlapped when the center of the circle is placed in the center of theshadow, and S represents the area of the shadow.

[0061] Further, the effective diameter is represented by MathematicalFormula 2.

Effective diameter=2(S/II)^(1/2)  (Mathematical Formula 2)

[0062] In the formula, II represents the circular constant.

[0063] Examples of the spherical anode active material include amesophase microbead formed through separating a mesophase spheruleproduced in the liquid-phase carbonization process from a pitch matrix,a material formed through carbonazing a spherical polymer of a highmolecular weight resin, a material formed through molding tar or pitchin a spherical shape and oxidizing it, and then carbonizing it.Moreover, a material formed through granulating non-spherical carbonparticles to form them in a spherical shape in a secondary particlelevel may be used. To granulate the particles, for example, a wet methodin which the particles are agitated and rolled using a liquid includinga solvent or a granulating aid to granulate, or a dry method in whichthe particles are rolled without any additives to granulate can be used.Further, a material formed through pulverizing non-spherical carbonparticles to form them in a spherical shape may be used.

[0064] In order for the mixture layer 22 b to have the above liquidabsorption properties, a material which resists crushing and maintainsair gaps between particles without eliminating them is preferablyincluded as the powdered anode active material. As such an anode activematerial, a material with a modulus of volume elasticity of 14 GPa orover is preferable. Further, an anode active material may have a coatingwith a modulus of volume elasticity of 14 GPa or over in a portion of asurface thereof so that the whole anode active material has an averagemodulus of volume elasticity of 14 GPa or over.

[0065] Examples of the anode active material with a modulus of volumeelasticity of 14 GPa or over include amorphous carbon such asdiamond-like carbon, non-graphitizable carbon, tin (Sn) and zinc (Zn).As the material of the coating with a modulus of volume elasticity of 14GPa or over, in order not to reduce the energy density, a material witha capacity by insertion and extraction of lithium is more preferable,but it is not limited to this. In addition to the above material, forexample, various kinds of transition metal such as copper and nickel,various transition metal compounds such as aluminum oxide (Al₂O₃) andtitanium oxide (TiO₂) may be used. As a method of coating the surface ofthe anode active material with the material, any of well-known methodscan be used. For example, the material may be added while forming slurrywhich will be described later, or may be sintered after dry or wetsupporting, or vapor deposition, CVD (chemical vapor deposition) or thelike may be used.

[0066] Moreover, in order for the mixture layer 22 b to have the aboveliquid absorption properties, as the powdered anode active material, amaterial having a thin hole through a powder so as to impregnate thematerial with the electrolyte solution is preferably included. Thecontent of the anode active material with a through hole in the mixturelayer 22 b is preferably 50 wt % or over, because a path where theelectrolyte solution penetrates can be sufficiently secured.

[0067] Further, as a method of forming a thin through hole, for example,a well-known method such as a granulating method, or a method in which amaterial capable of being removed such as pitch which is vaporized anddesorbed during heat treatment according to carbonization is added, andthen removed can be used. For example, in the case of the granulatingmethod, a wet method in which the particles are agitated and rolledusing a liquid including a solvent or a granulating aid to granulate, ora dry method in which the particles are rolled without any additives togranulate can be used.

[0068] The separator 23 is made of, for example, a porous film of asynthetic resin such as polytetrafluoroethylene, polypropylene orpolyethylene, or a porous film of ceramic, and the separator 23 may havea structure in which two or more kinds of the porous films arelaminated. Among them, a porous film made of polyolefin is preferablyused, because by use of the porous film, a short circuit can beeffectively prevented, and the safety of the battery can be improved bya shutdown effect. More specifically, polyethylene can obtain a shutdowneffect within a range of from 100° C. to 160° C., and is superior inelectrochemical stability, so polyethylene is preferably used as thematerial of the separator 23. Moreover, polypropylene is also preferablyused, and any other resin having chemical stability can be used bycopolymerizing or blending with polyethylene or polypropylene.

[0069] The porous film made of polyolefin is obtained through thefollowing steps, for example. After a molten polyolefin composite iskneaded with a molten low-volatile solvent in liquid form to form asolution uniformly containing a high concentration of the polyolefincomposite, the solution is extruded through a die, and is cooled to forma gel-form sheet, then the gel-form sheet is drawn to obtain the porousfilm.

[0070] As the low-volatile solvent, for example, a low-volatilealiphatic group such as nonane, decane, decalin, p-xylene, undecane orliquid paraffin, or a cyclic hydrocarbon can be used. A compositionratio of the polyolefin composite and the low-volatile solvent ispreferably 10 wt % to 80 wt % of the polyolefin composite, and morepreferably 15 wt % to 70 wt % of the polyolefin composite, when thetotal ratio of the polyolefin composite and the low-volatile solvent is100 wt %. When the composition ratio of the polyolefin composite is toosmall, during formation, swelling or neck-in becomes large at the exitof the die, so it is difficult to form the sheet. On the other hand,when the composition ratio of the polyolefin composite is too large, itis difficult to prepare a uniform solution.

[0071] When the solution containing a high concentration of thepolyolefin composite is extruded through the die, in the case of a sheetdie, a gap preferably has, for example, 0.1 mm to 5 mm. Moreover, it ispreferable that an extrusion temperature is within a range of from 140°C. to 250° C., and an extrusion speed is within a range of from 2cm/minute to 30 cm/minute.

[0072] The solution is cooled to at least a gelling temperature or less.As a cooling method, a method of directly making the solution contactwith cooling air, cooling water, or any other cooling medium, a methodof making the solution contact with a roll cooled by a cooling medium orthe like can be used. Moreover, the solution containing a highconcentration of the polyolefin composite which is extruded from the diemay be pulled before or during cooling at a pulling ratio of from 1 to10, preferably from 1 to 5. It is not preferable to pull the solution ata too large pulling ratio, because neck-in becomes large, and a rupturetends to occur during drawing.

[0073] It is preferable that, for example, the gel-form sheet is heated,and then is biaxially drawn through a tenter process, a roll process, arolling process, or a combination thereof. At this time, eithersimultaneous drawing in all direction or sequential drawing may be used,but simultaneous secondary drawing is preferable. The drawingtemperature is preferably equivalent to or lower than a temperature of10° C. higher than the melting point of the polyolefin composite, andmore preferably a crystal dispersion temperature or over and less thanthe melting point. A too high drawing temperature is not preferable,because effective molecular chain orientation by drawing cannot beachieved due to melting of the resin, and when the drawing temperatureis too low, softening of the resin is insufficient, thereby a rupture ofthe gel-form sheet tends to occur during drawing, so the gel-form sheetcannot be drawn at a high enlargement ratio.

[0074] After drawing the gel-form sheet, the drawn film is preferablycleaned with a volatile solvent to remove the remaining low-volatilesolvent. After cleaning, the drawn film is dried by heating or airblasting to volatilize the cleaning solvent. As the cleaning solvent,for example, an easily volatile material, that is, a hydrocarbon such aspentane, hexane or heptane, a chlorinated hydrocarbon such as methylenechloride or carbon tetrachloride, a fluorocarbon such astrifluoroethane, ether such as diethyl ether or dioxane is used. Thecleaning solvent is selected depending upon the used low-volatilesolvent, and one kind selected from the cleaning solvents or a mixturethereof is used. A method of immersing in the volatile solvent toextract, a method of sprinkling the volatile solvent, or a combinationthereof can be used for cleaning. Cleaning is performed until theremaining low-volatile solvent in the drawn film becomes less than 1part by mass relative to 100 parts by mass of the polyolefin composite.

[0075] The electrolyte solution with which the separator 23 isimpregnated includes a liquid solvent, for example, a nonaqueous solventsuch as an organic solvent or the like, and a lithium salt which is anelectrolyte salt dissolved in the nonaqueous solvent. The liquidnonaqueous solvent is made of, for example, a nonaqueous compound, andhas an intrinsic viscosity of 10.0 mPa·s or less at 25° C. Thenonaqueous solvent with an intrinsic viscosity of 10.0 mPa·s or less ina state that the electrolyte salt is dissolved therein may be used, andin the case where a plurality of kinds of nonaqueous compounds are mixedto form a solvent, the solvent may have an intrinsic viscosity of 10.0mPa·s or less in a state that the compounds are mixed. As such anonaqueous solvent, it is preferable that a high dielectric solvent withrelatively high dielectric constant is mainly used, and further amixture including a plurality of solvents with low viscosity is used.

[0076] Examples of the high dielectric solvent include ethylenecarbonate, propylene carbonate, butylene carbonate, vinylene carbonate,sulfolane, γ-butyrolactone and kinds of valerolactone, and one kind or amixture including two or more kinds selected from them may be used.

[0077] Examples of the solvent with low viscosity include symmetricchain carbonates such as diethyl carbonate and dimethyl carbonate,asymmetric chain carbonate such as methyl ethyl carbonate and methylpropyl carbonate, carboxylate such as methyl propionate and ethylpropionate, phosphate such as trimethyl phosphate and triethylphosphate, and one kind or a mixture including two kinds selected fromthem may be used.

[0078] Moreover, in addition to the above solvents, vinylene carbonate,trifluoropropylene carbonate, 1,2-dimethoxyethane, 1,2-diethoxymethane,tetrahydrofuran, 2-methyltetrahydrofuran, 4-methyl-1,3-dioxolane,sulfolane, methylsulfolane, 2,4-difluoroanisole, 2,6-difluoroanisole orthe like is preferably used, because battery characteristics can beimproved. The content thereof in the nonaqueous solvent is preferably 40vol % or less, more preferably 20 vol % or less.

[0079] Examples of the lithium salt include LiPF₆, LiClO₄, LiAsF₆,LiBF₄, LiB(C₆H₅)₄, CH₃SO₃Li, CF₃SO₃Li, LiN(CF₃SO₂)₂, LiC(CF₃SO₂)₃, LiCland LiBr, and one kind or a mixture including two or more kinds selectedfrom them may be used. When a mixture including two or more kinds isused, LiPF₆ is preferably used as a main component, because LiPF₆ hashigh conductivity and superior oxidation stability.

[0080] The content (concentration) of the lithium salt in a solvent ispreferably within a range of 0.5 mol/kg to 3.0 mol/kg. When the contentis out of the range, there may be cases where sufficient batterycharacteristics cannot be obtained due to an extreme decline in ionicconductivity.

[0081] Instead of the electrolyte solution, an electrolyte in which aretaining body made of a high molecular weight compound or an inorganiccompound retains an electrolyte solution may be used. The components ofthe electrolyte solution (that is, a solvent and an electrolyte salt)are as described above. Examples of the high molecular weight compoundinclude polyacrylonitrile, polyvinylidene fluoride, a copolymer ofpolyvinylidene fluoride and polyhexafluoropropylene,polytetrafluoroethylene, polyhexafluoropropylene, polyethylene oxide,polypropylene oxide, polyphosphazene, polysiloxane, polyvinyl acetate,polyvinyl alcohol, polymethylmethacrylate, polyacrylic acid,polymethacrylic acid, styrene-butadiene rubber, nitrile-butadienerubber, polystyrene and polycarbonate. Specifically, in terms ofelectrochemical stability, a high molecular weight compound having thestructure of polyacrylonitrile, polyvinylidene fluoride,polyhexafluoropropylene or polyethylene oxide is preferably used. Anamount of the high molecular weight compound added to the electrolytesolution varies depending upon compatibility between them, however, ingeneral, an amount of the high molecular weight compound equivalent to 5wt % to 50 wt % of the electrolyte solution is preferably added.

[0082] The secondary battery can be manufactured through the followingsteps, for example.

[0083] At first, for example, the cathode active material, an electronicconductor, and a binder are mixed to prepare a cathode mixture, and thecathode mixture is dispersed in a solvent such as N-methyl-2-pyrrolidoneto produce cathode mixture slurry in paste form. After the cathodemixture slurry is applied to the current collector 21 a, and the solventis dried, the mixture layer 21 b is formed through compression moldingby a roller press or the like so as to form the cathode 21.

[0084] Next, for example, an anode material capable of inserting andextracting lithium and a binder are mixed to prepare an anode mixture,then the anode mixture is dispersed in a solvent such asN-methyl-2-pyrrolidone to produce anode mixture slurry in paste form.After the anode mixture slurry is applied to the current collector 22 a,and the solvent is dried, the mixture layer 22 b is formed throughcompression molding by a roller press or the like so as to form theanode 22. At this time, the circularity, the modulus of volumeelasticity, the presence or absence of the through hole and the like inthe anode material capable of inserting and extracting lithium areadjusted to control the liquid absorption properties of the mixturelayer 22 b.

[0085] Then, the cathode lead 25 is attached to the current collector 21a by welding or the like, and the anode lead 26 is attached to thecurrent collector 22 a by welding or the like. After that, for example,a laminate including the cathode 21 and the anode 22 with the separator23 in between is spirally wound, and an end portion of the cathode lead25 is welded to the safety valve mechanism 15, and an end portion of theanode lead 26 is welded to the battery can 11. Then, the spirally woundlaminate including the cathode 21 and the anode 22 sandwiched between apair of insulating plates 12 and 13 is contained in the battery can 11.After the spirally wound laminate including the cathode 21 and the anode22 is contained in the battery can 11, the electrolyte is injected intothe battery can 11, and the separator 23 is impregnated with theelectrolyte. After that, the battery cover 14, the safety valvemechanism 15 and the PTC device 16 are fixed in an opened end portion ofthe battery can 11 through caulking by the gasket 17. Thereby, thesecondary battery shown in FIG. 1 is formed.

[0086] The secondary battery works as follows.

[0087] In the secondary battery, when charge is carried out, lithiumions are extracted from the mixture layer 21 b, and are inserted intothe anode material capable of inserting and extracting lithium includedin the mixture layer 22 b through the electrolyte with which theseparator 23 is impregnated. When the charge further continues, in astate where the open circuit voltage is lower than the overchargevoltage, the charge capacity exceeds the charge capacity of the anodematerial capable of inserting and extracting lithium, and then lithiummetal begins to be precipitated on the surface of the anode materialcapable of inserting and extracting lithium. After that, until thecharge is completed, precipitation of lithium metal on the anode 22continues.

[0088] Next, when discharge is carried out, at first, the lithium metalprecipitated on the anode 22 is eluted as ions, and is inserted into themixture layer 21 b through the electrolyte with which the separator 23is impregnated. When the discharge further continues, lithium ionsinserted into the anode material capable of inserting and extractinglithium in the mixture layer 22 b are extracted, and are inserted intothe mixture layer 21 b through the electrolyte. Therefore, in thesecondary battery, the characteristics of the conventional lithiumsecondary battery and the lithium-ion secondary battery, that is, ahigher energy density and superior charge-discharge cyclecharacteristics can be obtained.

[0089] Specifically in the embodiment, the mixture layer 22 b of theanode 22 has the liquid absorption properties that when 1 dm³ ofpropylene carbonate is dropped at 23° C., the contact angle θ that themixture layer 22 b forms with the propylene carbonate drop P becomes 10degrees or less within 100 seconds, so the mixture layer 22 b isimpregnated with the electrolyte solution quickly and uniformly.Therefore, lithium metal is uniformly precipitated on the whole mixturelayer 22 b. Thereby, falling off of precipitated lithium due tononuniform precipitation of the lithium metal or a loss of lithium dueto a reaction between the precipitated lithium and the electrolytesolution can be prevented.

[0090] Thus, in the embodiment, the mixture layer 22 b has the liquidabsorption properties that when 1 μdm³ of propylene carbonate is droppedat 23° C., the contact angle θ that the mixture layer 22 b forms withthe propylene carbonate drop P becomes 10 degrees or less within 100seconds, so the mixture layer 22 b is impregnated with the electrolytesolution quickly and uniformly. Thereby, lithium metal can be uniformlyprecipitated on the whole mixture layer 22 b. Therefore, whilemaintaining a high energy density, superior charge-discharge cyclecharacteristics can be obtained.

[0091] Accordingly, the battery can contribute to reduction in size andweight of portable electric devices typified by cellular phones, PDAs orlaptop computers.

[0092] Moreover, in the embodiment, the average circularity of theshadow of the anode active material is 0.7 or over, the anode activematerial with a modulus of volume elasticity of 14 GPa or over or theanode active material having a coating with a modulus of volumeelasticity of 14 GPa in at least a portion of the surface is used, orthe content of the anode active material with a through hole in themixture layer 22 b is 50 wt % or over, so the anode 22 according to theembodiment can be easily obtained.

[0093] Further, in the embodiment, the secondary battery in which thecapacity of the anode 22 is represented by the sum of a capacitycomponent by insertion and extraction of lithium and a capacitycomponent by precipitation and dissolution of lithium is described as anexample, but the anode 22 according to the embodiment can be applied toa secondary battery with any other structure.

[0094] As the secondary battery with the other structure, for example, aso-called lithium-ion secondary battery in which the capacity of theanode is represented by a capacity component by insertion and extractionof lithium is cited. For example, the lithium-ion secondary battery hasthe same structure as the above-described secondary battery, except thatthe amount of an anode material capable of inserting and extractinglithium is relatively large, compared to the cathode active material, soduring charge, lithium metal is not participated on the anode.

[0095] In the lithium-ion secondary battery, it is required to insertlithium ions into the anode, then to disperse the ions in a solid untilthe lithium-ion secondary battery is fully charged, so the speed ofimpregnating the anode 22 with the electrolyte solution is not limitedto a reacting portion of the anode, unless the speed is impracticallyslow. However, when the mixture layer can be impregnated with theelectrolyte solution quickly and uniformly, an electrode reaction can besufficiently carried out. Therefore, also in the lithium-ion secondarybattery, when the mixture layer of the electrode has the liquidabsorption properties that when 1 μdm³ of propylene carbonate is droppedon the mixture layer at 23° C., the contact angle that the mixture layerforms with the propylene carbonate drop becomes 10 degrees or lesswithin 100 seconds, while maintaining a high energy density, batterycharacteristics such as charge-discharge cycle characteristics can beimproved.

[0096] Next, specific examples of the invention will be described indetail below.

EXAMPLE 1-1 THROUGH 1-18

[0097] At first, 90 wt % of a powdered anode material capable ofinserting and extracting lithium as the anode active material and 10 wt% of polyvinylidene fluoride as the binder were mixed to form an anodemixture. At that time, as the anode material capable of inserting andextracting lithium, in Examples 1-1 through 1-6, artificial graphitewith an electrochemical equivalent of 512 mAh/cm³ in a insertionreaction of lithium, and a modulus of volume elasticity of 11.0 GPa wasused, and the average circularity of the shadow was 0.75 in Examples 1-1through 1-4 and 0.65 in Examples 1-5 and 1-6. The artificial graphitewas obtained as follows. A molded body was formed through kneading andhardening a filler which was a material of graphitizing coke particleswith a pitch binder, and then the molded body was graphitized at 3000°C. to obtain the artificial graphite. Moreover, in Examples 1-7 through1-10, natural graphite with a modulus of volume elasticity of 14.5 GPaas the whole powder and an average circularity of the shadow of 0.60 wasused. The natural graphite was obtained through coating the surface ofnatural graphite with a modulus of volume elasticity of 13.5 GPa and anelectrochemical equivalent of 576 mAh/cm³ in an insertion reaction oflithium with a furfuryl alcohol resin, and then sintering the naturalgraphite at 1000° C. Further, in Examples 1-11 and 1-12, naturalgraphite with a modulus of volume elasticity of 13.5 GPa, an averagecircularity of the shadow of 0.60, and an electrochemical equivalent of576 mAh/cm³ in an insertion reaction of lithium was used. In addition,Examples 1-13 through 1-18, a mixture including artificial graphite witha through hole, an average circularity of the shadow of 0.60 and amodulus of volume elasticity of 11.0 GPa and artificial graphite with nothrough hole, an average circularity of the shadow of 0.60 and a modulusof volume elasticity of 11.0 GPa was used. The artificial graphite witha through hole was formed through increasing the amount of the pitchbinder more than that in Examples 1-1 through 1-6 to have a larger thinhole, thereby making the thin hole through a particle. The content ofthe powdered anode active material with a through hole in the mixturelayer 22 b was 55 wt % in Examples 1-13 through 1-16 and 45 wt % inExamples 1-17 and 1-18. The presence or the absence of the thin hole waschecked through observing a sectional surface of the particle with anelectron microscope.

[0098] Next, the anode mixture was dispersed in N-methyl-2-pyrrolidoneas a solvent to form cathode mixture slurry in paste form. After thecathode mixture slurry was uniformly applied to both sides of thecurrent collector 22 a made of strip-shaped copper foil with a thicknessof 15 μm, and was dried. Then, the mixture layer 22 b was formed throughcompression molding with a certain pressure so as to form the anode 22with the total thickness of 160 μm. At that time, the pressure waschanged in Examples 1-1 through 1-18 to change the volume density of themixture layer 22 b as shown in Tables 1 through 3.

[0099] In the anode 22 of each of Examples 1-1 through 1-18, the liquidabsorption properties of the mixture layer 22 b were checked. Morespecifically, 1 μdm³ of propylene carbonate was taken with a syringe,and was dropped on the mixture layer 22 b of the anode 22. Then, thetime until the contact angle θ that the mixture layer 22 formed with thepropylene carbonate became 10 degrees or less at 23° C. (that is, liquidabsorption time) was measured with a stopwatch. The results are shown inTables 1 through 3 and FIGS. 4 through 6.

[0100] As Comparative Examples 1-1 and 1-2 relative to Examples 1-1through 1-6, anodes were formed as in the case of Examples 1-1 through1-6, except that the average circularity of the shadow of the anodematerial capable of inserting and extracting lithium or the volumedensity of the mixture layer was changed. As Comparative Examples 1-3and 1-4 relative to Examples 1-7 through 1-12, anodes were formed as inthe case of Examples 1-7 through 1-12, except that natural graphite inwhich the modulus of volume elasticity of the anode material capable ofinserting and extracting lithium was 13.5 GPa and an electrochemicalequivalent in an insertion reaction of lithium was 576 mAh/cm³ was used,and the volume density of the mixture layer was changed. Further, asComparative Examples 1-5 and 1-6 relative to Examples 1-13 through 1-18,anodes were formed as in the case of Examples 1-13 through 1-18, exceptthat the content of the powdered anode active material with a throughhole, or the volume density of the mixture layer 22 b was changed. Inthe anodes of Comparative Examples 1-1 through 1-6, as in the case ofExamples 1-1 through 1-18, the liquid absorption properties werechecked. The obtained results are shown in Tables 1 through 3 and FIGS.4 and 6.

[0101] It was obvious from Tables 1 through 3 and FIGS. 4 through 6 thatwhen the volume density was 1.6 g/cm³ or over, the liquid absorptionproperties sharply declined, but when the average circularity of theshadow of the powdered anode active material was 0.7 or over, when themodulus of volume elasticity of the powdered anode active material was14 GPa or over, or when the content of the powdered anode activematerial with a through hole in the mixture layer 22 b was 50 wt % orover, the decline could be prevented, and even if the volume density was1.8 g/cm³ or over, the time until the contact angle θ that the anode 22formed with 1 μdm³ of propylene carbonate became 10 degrees or less at23° C. could be within 100 seconds.

[0102] Moreover, a lithium-ion secondary battery in which the capacityof the anode 22 was represented by a capacity component by insertion andextraction of lithium was formed using the anode 22 of each of Examples1-1 through 1-18 and Comparative Examples 1-1 through 1-6. The shape ofthe battery was cylindrical as shown in FIGS. 1 and 2.

[0103] At that time, the cathode 21 was formed through the followingsteps. At first, lithium carbonate (Li₂CO₃) and cobalt carbonate (CoCO₃)were mixed at a ratio (molar ratio) of Li₂CO₃:CoCO₃=0.5:1, and themixture was fired in air at 900° C. for 5 hours to obtain lithium-cobaltcomplex oxide as the cathode material. When the X-ray diffraction of theobtained lithium-cobalt complex oxide was measured, the diffractionpattern closely matched a peak of LiCoO₂ listed in the JCPDS file. Next,the lithium.cobalt complex oxide was pulverized into the form of apowder with a particle diameter of 15 μm at 50% cumulative size whichwas obtained by a laser diffraction method to form a cathode activematerial.

[0104] Next, 95 wt % of lithium.cobalt complex oxide powder and 5 wt %of lithium carbonate powder were mixed to prepare a mixture, and thenthe 94 wt % of the mixture, 3 wt % of ketjen black as an electronicconductor and 3 wt % of polyvinylidene fluoride as a binder were mixedto prepare a cathode mixture. After the cathode mixture was prepared,the cathode mixture was dispersed in N-methyl-2-pyrrolidone as a solventto produce cathode mixture slurry in paste form. After the cathodemixture slurry was uniformly applied to both sides of the currentcollector 21 a made of strip-shaped aluminum foil with a thickness of 20μm, and was dried, the mixture layer 21 b was formed through compressionmolding by a roller press so as to form the cathode 21 with the totalthickness of 150 μm.

[0105] The used electrolyte solution was formed through dissolving aLiPF₆ content of 1.5 mol/dm³ in a solvent in which ethylene carbonateand dimethyl carbonate was mixed with equivalent volume. As theseparator 23, a stretched microporous polyethylene film with a thicknessof 27 μm was used. The outside diameter of the spirally wound electrodebody 20 was approximately a little more than 13 mm, and the size of thebattery was 14 mm in diameter and 65 mm in height.

[0106] A charge-discharge test was carried out on the battery todetermine a rated discharge capacity, a rated energy density and adischarge capacity retention ratio. At that time, charge was carried outat a constant current of 400 mA until a battery voltage reached 4.2 V,then the charge was continued at a constant voltage of 4.2 V until acharge time reached 4 hours. Discharge was carried out at a constantcurrent of 400 mA until the battery voltage reached 2.75 V. Thedischarge capacity in the second cycle was considered as the rateddischarge capacity, and the energy density was calculated by the value.The discharge capacity retention ratio was determined as a ratio of thedischarge capacity in the 300th cycle to the discharge capacity in thesecond cycle, that is, (discharge capacity in the 300 cycle)/(dischargecapacity in the second cycle)×100. In Tables 4 through 6, results ofExamples 1-3, 1-4, 1-9, 1-10, 1-15 and 1-16 are shown together with theresults of Comparative Examples 1-1 through 1-6.

[0107] It was obvious from Tables 4 through 6 that in Examples 0.1-3,1-4, 1-9, 1-10, 1-15 and 1-16 in which the liquid absorption time was100 seconds or less, the discharge capacity retention ratio could be ashigh as 78% or over, and even if the volume density was increased, thedischarge capacity retention ratio hardly declined. On the other hand,in Comparative Examples 1-1 through 1-6 in which the liquid absorptiontime was longer than 100 seconds, the discharge capacity retention ratiowas as low as 74% or less, and in Comparative Examples 1-3, 1-4, 1-5 and1-6, when the volume density was increased, the discharge capacityretention ratio extremely declined. In other words, it was found outthat when the mixture layer 22 b had the liquid absorption propertiesthat when 1 dm³ of propylene carbonate was dropped on the mixture layer22 b at 23° C., the contact angle that the mixture layer 22 b formedwith the propylene carbonate drop P became 10 degrees or less within 100seconds, while maintaining a high battery capacity and a high energydensity, the charge-discharge cycle characteristics could be improved.

EXAMPLES 2-1 THROUGH 2-6

[0108] A Battery in which the capacity of the anode 22 was representedby the sum of a capacity component by insertion and extraction oflithium and a capacity component by precipitation and dissolution oflithium was formed using the same anode 22 as that of each of Examples1-3, 1-4, 1-9, 1-10, 1-15 and 1-16 and Comparative Examples 1-1 through1-6. The battery was the same as that of Example 1-3, except that thetotal thickness of the anode 22 was 120 μm.

[0109] A charge-discharge test was carried out on the secondarybatteries of Examples 2-1 through 2-6 and Comparative Examples 2-1through 2-6 as in the case of Example 1-3 to determine the rateddischarge capacity, the rated energy density and the discharge capacityretention ratio. The obtained results are shown in Tables 7 through 9.

[0110] It was obvious from Tables 7 through 9 that in Examples 2-1through 2-6 in which the liquid absorption time was 100 seconds or less,the discharge capacity retention ratio could be as high as 71% or over,and even if the volume density was increased, the discharge capacityretention ratio hardly declined. On the other hand, in ComparativeExamples 2-1 through 2-6 in which the liquid absorption time was longerthan 100 seconds, the discharge capacity retention ratio was as low as68% or less, and when the volume density was increased, the dischargecapacity retention ratio extremely declined. In other words, it wasfound out that in the secondary battery in which the capacity of theanode 22 was represented by the sum of the capacity component byinsertion and extraction of lithium and the capacity component byprecipitation and dissolution of lithium, when the mixture layer 22 bhad the liquid absorption properties that when 1 μdm³ of propylenecarbonate was dropped on the mixture layer 22 b at 23° C., the contactangle θ that the mixture layer 22 b formed with the propylene carbonatedrop P became 10 degrees or less within 100 seconds, while maintaining ahigh battery capacity and a high energy density, the charge-dischargecycle characteristics could be improved.

[0111] The present invention is described referring to the embodimentand the examples, but the invention is not limited to the aboveembodiment and the examples, and is variously modified. For example, inthe above embodiments and the above examples, the anode 22 has theliquid absorption properties that when 1 μdm³ of propylene carbonate isdropped on the mixture layer 22 b of the anode 22 at 23° C., the contactangle θ that the mixture layer 22 b forms with the propylene carbonatedrop P becomes 10 degrees or less within 100 seconds, but the cathode 21or both of the cathode 21 and the anode 22 may have the above liquidabsorption properties.

[0112] Moreover, in the embodiment and the examples, the case wherelithium is used as light metal is described; however, the invention canbe applied to the case where any other alkali metal such as sodium (Na)and potassium (K), alkaline-earth metal such as magnesium and calcium(Ca), any other light metal such as aluminum, an alloy of lithium, or analloy thereof is used, thereby the same effects can be obtained. In thiscase, the anode material capable of inserting and extracting lightmetal, the cathode active material, the nonaqueous solvent, theelectrolyte salt or the like is selected depending upon the light metal.However, lithium or an alloy including lithium is preferably used as thelight metal, because voltage compatibility with lithium-ion secondarybatteries which are practically used at present is high. Further, whenthe alloy including lithium is used as the light metal, a materialcapable of forming an alloy with lithium may be present in theelectrolyte or the anode so as to form an alloy during precipitation.

[0113] Further, in the embodiments and the examples, the cylindricaltype secondary battery with a spirally wound structure is described;however, the invention is applicable to an elliptic type or a polygonaltype secondary battery with a spirally wound structure, or a secondarybattery with a structure in which the cathode and the anode are foldedor laminated in a like manner. In addition, the invention is applicableto a secondary battery with a coin shape, a button shape, a prismaticshape or the like. Further, the invention is applicable to not only thesecondary batteries but also primary batteries.

[0114] As described above, in the electrode or the battery according tothe invention, the mixture layer has the liquid absorption propertiesthat when 1 μdm³ of propylene carbonate is dropped on the mixture layerat 23° C., the contact angle that the mixture layer forms with thepropylene carbonated drop is 10 degrees or less within 100 seconds, sothe mixture layer can be impregnated with the electrolyte solutionquickly and uniformly. Therefore, while maintaining a high energydensity, superior charge-discharge cycle characteristics can beobtained. TABLE 1 CONTENT OF ANODE ACTIVE MATERIAL MODULUS OF WITHLIQUID VOLUME THROUGH VOLUME ABSORPTION AVERAGE ELASTICITY HOLE DENSITYTIME CIRCULARITY (GPa) (wt %) (g/cm³) (sec.) EXAMPLE 1-1 0.75 11.0 0 1.218 EXAMPLE 1-2 0.75 11.0 0 1.4 28 EXAMPLE 1-3 0.75 11.0 0 1.6 76 EXAMPLE1-4 0.75 11.0 0 1.8 97 EXAMPLE 1-5 0.65 11.0 0 1.2 25 EXAMPLE 1-6 0.6511.0 0 1.4 41 COMPARATIVE 0.65 11.0 0 1.6 264 EXAMPLE 1-1 COMPARATIVE0.65 11.0 0 1.8 1151 EXAMPLE 1-2

[0115] TABLE 2 CONTENT OF ANODE ACTIVE MATERIAL MODULUS OF WITH LIQUIDVOLUME THROUGH VOLUME ABSORPTION AVERAGE ELASTICITY HOLE DENSITY TIMECIRCULARITY (GPa) (wt %) (g/cm³) (sec.) EXAMPLE 1-7 0.60 14.5 0 1.2 19EXAMPLE 1-8 0.60 14.5 0 1.4 31 EXAMPLE 1-9 0.60 14.5 0 1.6 53 EXAMPLE1-10 0.60 14.5 0 1.8 91 EXAMPLE 1-11 0.60 13.5 0 1.2 28 EXAMPLE 1-120.60 13.5 0 1.4 39 COMPARATIVE 0.60 13.5 0 1.6 127 EXAMPLE 1-3COMPARATIVE 0.60 13.5 0 1.8 951 EXAMPLE 1-4

[0116] TABLE 3 CONTENT OF ANODE ACTIVE MATERIAL MODULUS OF WITH LIQUIDVOLUME THROUGH VOLUME ABSORPTION AVERAGE ELASTICITY HOLE DENSITY TIMECIRCULARITY (GPa) (wt %) (g/cm³) (sec.) EXAMPLE 1-13 0.60 11.0 55 1.2 15EXAMPLE 1-14 0.60 11.0 55 1.4 34 EXAMPLE 1-15 0.60 11.0 55 1.6 56EXAMPLE 1-16 0.60 11.0 55 1.8 81 EXAMPLE 1-17 0.60 11.0 45 1.2 24EXAMPLE 1-18 0.60 11.0 45 1.4 35 COMPARATIVE 0.60 11.0 45 1.6 105EXAMPLE 1-5 COMPARATIVE 0.60 11.0 45 1.8 427 EXAMPLE 1-6

[0117] TABLE 4 STRUCTURE OF ANODE DISCHARGE LIQUID RATED RATED CAPACITYVOLUME ABSORPTION TOTAL DISCHARGE ENERGY RETENTION AVERAGE DENSITY TIMETHICKNESS CAPACITY DENSITY RATIO CIRCULARITY (g/cm³) (sec.) (μm) (mAh)(Wh/l) (%) EXAMPLE 1-3 0.75 1.6 76 160 857.2 325.7 81 EXAMPLE 1-4 0.751.8 97 160 857.2 338.6 80 COMPARATIVE 0.65 1.6 264 160 856.6 325.5 52EXAMPLE 1-1 COMPARATIVE 0.65 1.8 1151 160 856.6 338.4 54 EXAMPLE 1-2

[0118] TABLE 5 STRUCTURE OF ANODE DISCHARGE MODULUS OF LIQUID RATEDRATED CAPACITY VOLUME VOLUME ABSORPTION TOTAL DISCHARGE ENERGY RETENTIONELASTICITY DENSITY TIME THICKNESS CAPACITY DENSITY RATIO (GPa) (g/cm³)(sec.) (μm) (mAh) (Wh/l) (%) EXAMPLE 1-9 14.5 1.6 53 160 892.5 339.1 79EXAMPLE 1-10 14.5 1.8 91 160 927.8 352.5 78 COMPARATIVE 13.5 1.6 127 160891.2 338.6 54 EXAMPLE 1-3 COMPARATIVE 13.5 1.8 951 160 926.5 352.0 35EXAMPLE 1-4

[0119] TABLE 6 STRUCTURE OF ANODE CONTENT OF ANODE ACTIVE MATERIALDISCHARGE WITH LIQUID RATED RATED CAPACITY THROUGH VOLUME ABSORPTIONTOTAL DISCHARGE ENERGY RETENTION HOLE DENSITY TIME THICKNESS CAPACITYDENSITY RATIO (wt %) (g/cm³) (sec.) (μm) (mAh) (Wh/l) (%) EXAMPLE 1-1555.0 1.6 56 160 863.9 328.3 84 EXAMPLE 1-16 55.0 1.8 81 160 898.1 341.383 COMPARATIVE 45.0 1.6 105 160 861.3 327.3 74 EXAMPLE 1-5 COMPARATIVE45.0 1.8 427 160 895.4 340.3 66 EXAMPLE 1-6

[0120] TABLE 7 STRUCTURE OF ANODE DISCHARGE LIQUID RATED RATED CAPACITYVOLUME ABSORPTION TOTAL DISCHARGE ENERGY RETENTION AVERAGE DENSITY TIMETHICKNESS CAPACITY DENSITY RATIO CIRCULARITY (g/m³) (sec.) (μm) (mAh)(Wh/l) (%) EXAMPLE 2-1 0.75 1.6 76 120 946.1 356.6 73 EXAMPLE 2-2 0.751.8 97 120 973.4 366.9 71 COMPARATIVE 0.65 1.6 264 120 945.7 356.4 52EXAMPLE 2-1 COMPARATIVE 0.65 1.8 1151 120 973.0 366.7 34 EXAMPLE 2-2

[0121] TABLE 8 STRUCTURE OF ANODE DISCHARGE MODULUS OF LIQUID RATEDRATED CAPACITY VOLUME VOLUME ABSORPTION TOTAL DISCHARGE ENERGY RETENTIONELASTICITY DENSITY TIME THICKNESS CAPACITY DENSITY RATIO (GPa) (g/cm³)(sec.) (μm) (mAh) (Wh/l) (%) EXAMPLE 2-3 14.5 1.6 53 120 953.1 359.4 73EXAMPLE 2-4 14.5 1.8 91 120 980.6 369.9 71 COMPARATIVE 13.5 1.6 127 120952.2 358.8 54 EXAMPLE 2-3 COMPARATIVE 13.5 1.8 951 120 979.7 369.2 35EXAMPLE 2-4

[0122] TABLE 9 STRUCTURE OF ANODE CONTENT OF ANODE ACTIVE MATERIALDISCHARGE WITH LIQUID RATED RATED CAPACITY THROUGH VOLUME ABSORPTIONTOTAL DISCHARGE ENERGY RETENTION HOLE DENSITY TIME THICKNESS CAPACITYDENSITY RATIO (wt %) (g/cm³) (sec.) (μm) (mAh) (Wh/l) (%) EXAMPLE 2-555.0 1.6 56 120 947.5 357.1 77 EXAMPLE 2-6 55.0 1.8 81 120 974.9 367.474 COMPARATIVE 45.0 1.6 105 120 946.3 356.6 68 EXAMPLE 2-5 COMPARATIVE45.0 1.8 427 120 973.6 366.9 59 EXAMPLE 2-6

1. An electrode, comprising: a mixture layer including a powderedelectrode active material, wherein the mixture layer has liquidabsorption properties that when 1 μdm³ of propylene carbonate is droppedon the mixture layer at 23° C., a contact angle that the mixture layerforms with a propylene carbonate drop becomes 10 degrees or less within100 seconds.
 2. An electrode according to claim 1, wherein the averagecircularity of a shadow of the electrode active material is 0.7 or over.3. An electrode according to claim 1, wherein the electrode activematerial has a modulus of volume elasticity of 14 GPa or over, or theelectrode active material has a coating with a modulus of volumeelasticity of 14 GPa or over in at least a portion of a surface thereof.4. An electrode according to claim 1, wherein at least a portion of theelectrode active material has a through hole through a powder, and thecontent of the electrode active material with the through hole in themixture layer is 50 wt % or over.
 5. A battery, comprising: a cathode;an anode; and an electrolyte solution, wherein at least either thecathode or the anode comprises: a mixture layer including a powderedelectrode active material, wherein the mixture layer has liquidabsorption properties that when 1 g dm³ of propylene carbonate isdropped on the mixture layer at 23° C., a contact angle that the mixturelayer forms with a propylene carbonate drop becomes 10 degrees or lesswithin 100 seconds.
 6. A battery according to claim 5, wherein theaverage circularity of a shadow of the electrode active material is 0.7or over.
 7. A battery according to claim 5, wherein the electrode activematerial has a modulus of volume elasticity of 14 GPa or over, or theelectrode active material has a coating with a modulus of volumeelasticity of 14 GPa or over in at least a portion of a surface thereof.8. A battery according to claim 5, wherein at least a portion of theelectrode active material has a through hole through a powder, and thecontent of the electrode active material with the through hole in themixture layer is 50 wt % or over.
 9. A battery according to claim 5,wherein a capacity of the anode is represented by the sum of a capacitycomponent by insertion and extraction of light metal and a capacitycomponent by precipitation and dissolution of light metal, and themixture layer of the anode has the liquid absorption properties.